1. Field of the Invention
The present invention relates to medical imaging and, more particularly, to a method and apparatus for breast imaging using non-ionizing photons in the electromagnetic spectrum, such as optical energy.
2. Discussion of Related Art
According to the American Cancer Society, in 1996 184,300 women will be diagnosed with breast cancer and 44,300 women will die from breast cancer. The ACS estimates that 1,500,000 women will be diagnosed and more than 500,000 women will die of breast cancer in the 1990's. Early breast cancer detection increases the patient's chances of surviving the cancer. Thus, early detection is a major factor in saving the lives of breast cancer patients. The five-year survival rate is 96% for women having early detection of breast cancer. Although experts agree that a mammogram is the single best means of early breast cancer detection, in 60% of all women diagnosed with breast cancer, early detection was missed due to low-contrast mammography images.
1. Current Breast Imaging Methods
Mammography is the radiological examination of the human breast. It is generally accepted that mammography is an effective and reliable procedure in the early detection of breast cancer. Mammography is typically performed using x-ray or other traditional film/screen techniques. However, these techniques do not always provide adequately high-quality images to detect cancer, particularly in the relatively large population of women having radiodense breast tissue (younger women, for example, tend to have radiodense breast tissue). Mammograms require high-quality images because the tissue density between adipose (fatty), glandular, calcified, or cancerous tissue is less diverse than, for example, flesh and bone. Thus, "subtler" contrasts are required to distinguish between these types of tissue.
Traditional film mammograms do not provide these subtler contrasts. Film mammograms have a non-linear response to x-ray exposure. That is, for example, doubling the x-ray exposure of film or halving the breast density, does not result in an image that is twice as bright. As a result, a single traditional film x-ray exposure often does not show the entire tonal range of a patient's breast tissue. Often, a radiologist may take exposures at different energy levels to provide images with different contrasts. This exposes the patient to several doses of x-rays, which are a known cause of cancer.
Other drawbacks are caused by the poor contrast of film mammograms. One of these drawbacks is that it is difficult to detect masses in patient's having breast implants. A second drawback is that it is difficult to discern between benign and malignant microcalcifications and tumors. This latter drawback results in thousands of unnecessary invasive procedures to remove growths which are later determined to be benign. If a mammogram could allow a radiologist to distinguish more clearly between benign and malignant tissue, many of those procedures would be prevented.
FIG. 1 illustrates a conventional mammography machine using a traditional film technique. The conventional mammography machine 50 has an x-ray tube 52 which emits x-rays and an image receptor 54 which receives the x-ray radiation. During use, a breast 56 is compressed between a compression plate 58 which holds the breast in position, and a bucky tray 60, which sits on top of the image receptor, supports the breast 56 and houses a grid 62. The breast is typically compressed very tightly by the compression plate. The compression is applied to make the breast have as uniform a shape as possible to obtain an accurate image. In addition to being very uncomfortable for the patient, some experts believe that this compression may release existing cancer cells into the vascular system of women who may already have undetected breast cancer.
Beneath the grid is a scintillator 64 which generates visible light when excited by x-rays. A film 66 is placed below the bucky tray and scintillator and on top of a radiation detector 68 used for automatic exposure control, such as an ionization chamber or a solid state sensor. X-ray radiation passes through the breast tissue and strikes the scintillator 64. The scintillator generates visible light according to the x-ray striking it. The visible light enters the image receptor and exposes the film. After the film is exposed, it must be developed before it may be viewed. Areas on the developed film not exposed to the x-ray radiation result in a light area on the film indicating that the radiation did not pass through the tissue. This is an indication of a mass or other x-ray blocking body.
Digital mammography which employs a solid state electronic imager in place of film is an emerging technology in the detection of breast cancer. Digital mammography offers several advantages, such as shorter procedure times, improved image quality, decreased health care costs because digital mammograms provide early detection; and lower radiation exposure. Digital mammography also faces severe technical challenges. Soft-tissue breast imaging has the most stringent imaging requirements of all radiological imaging. Two reasons for this are the slight difference in densities between the tissue types found in breasts (adipose, glandular, calcified, and cancerous) and the relatively small size of breast tumors in their initial stages. As a result, mammography requires very fine pixel dimensions (e.g., less than 40 microns) and a high contrast dynamic range (14 bits, i.e., 2.sup.14 or 16,384 tone levels) over a large area (i.e., 24 cm.times.30 cm). The American College of Radiology recommends an image resolution of 11 to 13 line pairs per millimeter. This means that the image should be sharp enough to distinguish between 11 to 13 pairs of white and black lines in a one millimeter space.
In any event, digital mammography still requires uncomfortable breast compression and exposure to potentially harmful, cancer-causing x-ray radiation.
Magnetic Resonance Imaging (MRI) has also been used for breast imaging. Although MRI has been found to provide acceptable resolution images, an MRI machine is very expensive. The MRI machine is a limited resource which may best be used for other purposes. Breast imaging using MRI may be too expensive to be practical. This cost issue is vitally important. Experts currently disagree about the age at which a woman should begin having annual mammograms. Although some consideration of this debate involves the amount of radiation a woman is exposed to, a great concern is that of health care insurers (such as HMOs): health care insurers look for a prevention costs v. medical benefit costs "break even" point. If statistics show that it will cost more to pay for mammographies for all women of a certain age than the medical costs for medical treatment of women of that age expected to develop breast cancer, regular mammograms are not recommended. Although this may make economic sense, it does not make sense to the younger women at the lower end of the statistical analysis who have undetected breast cancer. Therefore, a more expensive alternative to conventional mammography may not result in increased detection, prevention, and saved lives.
2. Breast Imaging Using Non-Ionizing Light
Opto-electronics have been used for breast imaging. For example, near-red spectroscopy uses non-ionizing radiation which passes easily through breast tissue and may be tolerated in relatively large doses. Early efforts to provide optical breast imaging were unsuccessful because light scattering was not measurable and the resultant image quality was poor.
Some infrared (IR) imaging devices use IR cameras to detect "hot spots" in the breast, are hand-held devices, or require the injection of dyes into the patient. Imaging Diagnostics Systems Inc. of Plantation, Florida, has developed a laser tomography imaging device which uses a single laser which revolves around a patient's breast. Power, control, and output signals are exchanged from the rotating laser device to the rest of the machine using slip rings, which may be noisy; DC power supplies are mounted on the rotating device. The device includes no adjustability for different sized breasts.
Lasers have recently been considered as useful diagnostic tools because light travels differently depending upon what it travels through. Obstacles in the way of laser photons may cause the photons to scatter or be absorbed. For example, cancerous cells contain more hemoglobin than non-cancerous cells and absorb light more readily. Cells having a high density (another sign of cancer) tend to scatter photons. These scatter and absorption characteristics may vary depending on the wavelength of light used.
It is an object of the present invention to provide a method and device for breast imaging that does not use ionizing radiation, such as x-rays.
It is another object of the present invention to provide a breast imaging device that does not require the breast to be compressed.
It is yet another object of the present invention to provide a device which permits inexpensive breast imaging.
It is a further object of the present invention to provide a breast imaging device that can detect very small growths.
It is even a further object of the present invention to provide a breast imaging device that can distinguish between types of cell growths and thus reduce the need for unnecessary biopsy procedures.
It is yet a further object of the present invention to provide a breast imaging device that allows real-time imaging during surgical procedures, such as needle biopsies.